1. Field of the Invention
The present invention relates to a process for the production of maleic anhydride by catalytic air oxidation of suitable hydrocarbons.
2. Description of the Prior Art
In the air oxidation of hydrocarbons, such as benzene, butane, butene or butadiene to maleic anhydride and water at about 400.degree. C., mainly even-numbered lower carboxylic acids, carbon monoxide and carbon dioxide are formed as by-products. In order to recover as much maleic anhydride as possible directly in solid form from the oxidation stream, the air stream with the resulting substances is cooled, inter alia, to temperatures above the dew point of the accompanying water to prevent hydrolysis. In this way, it is possible to separate up to about 85% by weight of the resulting maleic anhydride immediately after benzene oxidation and up to about 30-50% by weight immediately after butane/butene oxidation. To recover the remaining amount, the gas stream containing the maleic anhydride is scrubbed with so much water that the anhydride is virtually completely precipitated. Thereby an approximately 40% maleic acid solution is formed by hydrolysis. In order to, again, recover maleic anhydride therefrom, according to the conventional wisdom in the art, inter alia, all the water is distilled off from the top of a suitable column by an expensive, energy-consuming process with the addition of entrainer, e.g., xylene, whereby the maleic anhydride is re-formed from the maleic acid. In addition, a small amount of fumaric acid is also formed by cis-trans-isomerization.
Of course, it is also possible to dispense entirely with the separation of the solid maleic anhydride by the corresponding cooling of the oxidation stream and instead of this, to scrub the total oxidation gas immediately after it leaves the reactor. This can be advantageous above all after oxidation of C.sub.4 gases, since because of the higher dew point of the water only a relatively small amount of maleic anhydride can be recovered directly by cooling of the oxidation stream. Unfortunately, this manner of operation is particularly costly in energy.
For this reason, many proposals have suggested the avoidance of the gas stream scrubbing with water. Thus in U.S. Pat. No. 3,198,680 it is recommended that the esters of phthalic acid with C.sub.4 -C.sub.6 alcohols be used as the scrubbing medium. EP-AS No. 00 19 046 uses cycloaliphatic acid esters as the scribbing medium. In DE-OS No. 2 444 824 dibenzylbenzene and in EP-PS No. 1 443 411 polymethylbenzophenones are proposed as scrubbing liquids.
Besides other disadvantages, the recommended solvents above all exhibit slight extraction effects especially under the operating conditions of the process, which are caused by the fact that the large oxidation gas stream largely removes the maleic anhydride from the organic medium.
Therefore, the extraction of the maleic anhydride with inert organic media in the air oxidation process has led only to a modest success. However, the carboxylic groups resulting from hydrolysis or alcoholysis of the maleic anhydride should bind intensively to suitable basic extracting agents. To be able to separate excess water, the extracting agent must not be soluble in it. Further, the bond with the maleic acid should break by a simple process--if possible merely by heating of the extract--and, after formation of the maleic anhydride, a clean separation from the resulting water should also occur. The extracting agent must be able to be reused.
A process described by M. I. Yakushkin (SU-PS No. 168 674) relates to the isolation of lower monocarboxylic acids such as formic, acetic, propionic and butyric acid, by extraction from aqueous solutions with water-insoluble trioctylamine. The acids can be released from the resulting salt after separation of the water by heat treatment.
It was then found that maleic acid, a dicarboxylic acid, can also be extracted to some extent from aqueous solutions with tri-n-octylamine. Unfortunately, tri-n-octylamine is insufficiently stable to atmospheric oxygen and presents severe drawbacks when used as an extracting agent for maleic anhydride. Moreover, the current methods in use for producing maleic anhydride suffer from low product yields.
Therefore, a need clearly continues to exist for a process for the production of maleic anhydride which uses an extracting agent which does not suffer from insufficient stability to air and which produces maleic anhydride in excellent yield.